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networking problems, including the limited number of disks where network storage can reside and bottlenecks in network traffic to the server. However, Fibre Channel can be too expensive and complex for basic network storage tasks.

That's when you may want network-attached storage (NAS). NAS servers offer accessibility and performance that falls between server storage and the SAN. NAS devices attach to the LAN through an ordinary Ethernet connection and IP address, providing disk storage as an independent network device (though NAS devices can also be added to a SAN). Multiple NAS "boxes" can be added to the LAN as needed, allowing for additional network storage as needed.

NAS hardware and architecture

A NAS box is basically a dedicated data storage server that includes a NAS head and disk drives attached to a network. The NAS head (a.k.a. NAS front end or NAS gateway) is the control electronics that interface the network and storage. A basic NAS server may use just one head or share the internal storage across multiple heads to accommodate increased network bandwidth.

In terms of storage, NAS server systems are typically denoted by their drive support, the total number of drives and the total capacity. SATA drives are easily the most commonly employed type of drive in NAS systems. SATA drives allow for low-cost high-density NAS storage, though ATA, SCSI, SATA and SAS drives may also be used in some models.

Workgroup-type NAS systems generally support at least 1 to 2 TB of disk storage with a small group of four to six hard disks, depending on the particular model and options selected, though some models support expansion disk racks that allow extended storage up to 30 TB. High-end enterprise-class NAS server systems can implement many disks and offer capacities well over 100 TB. Most NAS systems include RAID support for data protection and can implement common RAID levels, including RAID-0, RAID-1 and RAID-5. NAS systems also include some onboard memory (RAM) to cache network data to or from the disks. Small NAS devices may only provide a 128 MB to 256 MB cache, though large enterprise-class NAS systems may offer cache up to 4 GB.

A NAS box must actually connect to the LAN, so the network interface is also important. Standard 10/100 Ethernet support is common, but Gigabit Ethernet (GigE) ports are becoming more prevalent. Some NAS server products provide dual Ethernet connections for network interface redundancy or failover.

Consider the V800 workgroup NAS server system from Mpak Technologies as one example. The V800 supports up to eight ATA disks in RAID configurations to level 5. The unit includes up to 512 MB of cache and offers two 10/100 Ethernet ports -- GigE ports are available as an option) By comparison, the StorEdge 5310 NAS from Sun Microsystems Inc. natively supports 18 disks up to 2.6 TB and 4 GB of onboard cache, and includes two 10/100/1000 Ethernet ports for connectivity.

While NAS boxes typically work independently, they can also be aggregated into clusters. Similar in principle to clustered computing, NAS server clusters appear to the LAN as a single NAS device. Each clustered element can share the data load and each box in the cluster can provide failover if another box fails -- helping to improve storage performance and achieve very high storage availability. OnStor Inc. provides a clustered NAS gateway designed to interconnect Windows, Linux and Unix client systems with up to 40 petabytes of storage capacity from a variety of vendors.

NAS server protocols

Protocols are sets of rules that define the way that two end points communicate and NAS devices interact with the LAN using a variety of important protocols that manage activities like networking, file exchanges and applications.<'p>

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